This invention relates primarily to gas dynamic lasers and more particularly to means for perturbing the flow of gas passing through a supersonic nozzle.
The expansion of a fluid from subsonic to supersonic velocity is common in many fluid flow applications. During such an expansion even the most carefully designed aerodynamic nozzles produce some shock or expansion waves in the divergent section of the nozzle particularly at the extremities of two-dimensional nozzles. When a wave is produced in an expansion nozzle, or any other device, a pressure differential and a density variation of the working fluid results across the wave. A major cause of quality degradation of an output beam from a gas dynamic laser is density variations within the gas passing through the optical cavity produced by a shock-expansion waves emanating from intersections at the end and throat regions of a nozzle and the walls of the gas path channel. Imperfection in the nozzle mounting arrangements, changes in the boundary layer thicknesses along the walls of the gas path in the vicinity of the nozzle throat, secondary flow in the boundary layer along the walls of the nozzles, etc., can generate expansion waves which propagate into the optical cavity producing density variations within the gas passing therethrough which perturbs the phase front of the laser beam and degradates the quality of the output beam.
Various attempts have been made to eliminate the optical distortions in the output beam resulting from these disturbances, but none are considered adequate. One method of compensating for the effects of the optical phase disturbances produced by this density variation is the use of mirrors, capable of being deformed, at the ends of each optical beam pass through the cavity. During operation the surface contours of the mirrors are adjusted to produce an opposite phase disturbance to correct for the density variations. This approach is particularly difficult because of the complexity of the fabrication and maintenance of the mirrors as well as the complexity of controlling the corrections required for each specialized case.
McLafferty in U.S. Pat. No. 3,860,885 filed Mar. 13, 1973 and held with the present application by a common assignee discloses a supersonic expansion nozzle arrangement which is especially useful in high power flowing gas laser systems. The nozzle arrangement comprises a plurality of specially contoured two-dimensional elements which are combined in a staggered array upstream of the optical cavity. Sinch each element produces a characteristic shock and expansion wave pattern that radiates in the downstream direction, the staggered array prevents the individual wave patterns from each nozzle from reinforcing one another and the severity of the phase front variations in the optical cavity is reduced resulting in improvements to the optical quality of the output beam.